Surface forces and charging properties of toners are modified by application of surface treatments. The most common surface treatments are surface modified fumed silicas, but fine particles of titania, alumina, zinc oxide, tin oxide, cerium oxide, and polymer beads can also be used. Surface treatment may serve other functions such as providing cleaning aids to ancillary processing in an electrophotographic process. The function of reducing the forces is achieved by separation of the toner from other surfaces by the very small surface treatment particles. The attractive Van der Waals forces between toner particles and other surfaces decrease as (D/s)2 where D is the toner diameter and s is the separation at the closest point between the toner and the other surface and s<<D. A few points of contact between the other surface and the toner created by the surface treatment increase the separation between the surfaces. The contacts of the surface treatment with the toner and another surface add a small attractive force. As such, the ideal situation is for the surface treatment to be uniformly dispersed on the toner with a minimum coverage to affect the desired separation given the curvature of the toner and the size of the surface treatment.
The reduction of attractive forces exerted on a toner enhances processes where the toner particles must move. Some processes that benefit from lower adhesive and cohesive forces are toner powder flow in the replenisher, mixing of toner in the developer station, development of toner onto the latent image, transfer of the image to intermediate and final receivers and cleaning of residual images from photoconductors and intermediate receivers. In these processes, the attractive forces are overcome by gravity, mechanical, inertial and electrostatic forces. Often, some cohesive force of the toner is beneficial and an optimum separation of the toner particles from other surfaces exists. These other forces are used to move the toner from the developer to the latent image and transfer the image from the photoconductor to intermediate and final receivers. Like Van der Waals forces, electrostatic forces scale with D2. The other forces scale with D3 and are less effective at moving smaller toner hence the greater need to reduce the Van der Waals forces for smaller toner.
Toner is often exposed to violent collisions and shearing motion to induce a static charge on the toner, to develop latent images on photoreceptors with toner, to transfer the developed images to intermediate and final receivers, and in other ancillary processes involving toner such as cleaning. Violent collisions of the toner particle normal to the surface of the toner direct the impulse force on the surface treatment. The impulse force can exceed the strength of the toner core material (usually a melt adhesive polymer with a glass transition temperature, Tg, in the range of 50 to 60 degrees centigrade). The kinetic energy of the collision is transformed into heat and, because of the short duration of the collision event, the heat is localized at the surface treatment contact points with the toner particle and other surface. The local temperature at the contact briefly exceeds the Tg and the toner core material will plastically deform around the surface treatment increasing the area of contact. Because the separation in this area of contact is on the atomic scale, the attractive forces between the surface treatment and the toner are greatly increased. When this attractive force exceeds the shearing forces applied in the system, the surface treatment is tacked to the toner surface. The area of contact required to achieve a tacked state depends upon the chemistry of the toner and the surface treatment and is highly sensitive to chemical modifications of the outer surface of the surface treatment. Further impaction will continue to embed the toner and in extreme cases, the surface treatment will be pushed into the toner until it is flush with the surface. Further impaction works the toner core material in plastic deformation covering over and engulfing the surface treatment. As the surface treatment becomes increasingly embedded and engulfed, it is less effective at maintaining the desired separation between the toner particle and other surfaces.
Before the surface treatment becomes tacked, shearing motions may move its position on the toner surface. The movement reduces the spacing and may allow contact of the core material of the toner particle with another surface. With sufficient shearing, the surface treatment will be concentrated in low (concave) areas of the toner surface necessitating an initial excess of surface treatment to obtain the desired separation. During gentle collisions and shearing contacts, some of the surface treatment may transfer to other surfaces. This reduces the effectiveness of the surface treatment and may create problems associated with the other surface. For example, transfer of the surface treatment to the carrier surface in a two component system may change the internal coefficient of friction resulting changes in packed density and flow characteristics. Control of packed density of the developer is important because many toner concentration control algorithms rely upon changes in magnetic density as a function of toner concentration to measure the concentration for feedback control.
Tacking the surface treatment in place once uniformly dispersed on the toner surface under controlled conditions with low shear allows the use of lower surface treatment concentrations to achieve the desired separation. Tacking will also prevent transfer of the surface treatment to other surfaces. However, the tacking initiates the embedment process and reduces the number of impacts a toner particle may sustain before the surface treatment becomes ineffective at maintaining the desired separation from other surfaces. Large surface treatment may sustain many more impacts before embedment reduces the effectiveness. As the size of surface treatment particles increase, the area of contact increases and the energy collisions must increase to bring the localized temperature above the Tg required for increasing the degree of embedment.
Many surface treatments have several states of agglomeration. Deagglomeration and dispersion of silica is required in the manufacturing step of applying the surface treatment to the toner. Some agglomerates that can be dispersed in the toning process may be left as a reservoir to replace surface treatment that becomes lost due to engulfment or transfer to other surfaces. However, several problems exist with this approach. First, these agglomerates are rapidly lost to other surfaces and aggravate the problems described above. Second, the rate of deagglomeration is difficult to match with the rate of embedment. Third, agglomerates that have significant life times in the electrophotographic process are difficult to disperse uniformly on the toner surface in the manufacturing process. Last, large agglomerates will cause voids in the image.
The surface treatment may be tacked in place during manufacturing by high mechanical forces to induce sufficient temperature rise at the contact points between the surface treatment and the toner particle. Some mechanical devices generate the intense mechanical force by compressive shearing of a packed toner bed between a moving tools and a stationary wall. A high degree of shear rapidly heats the toner increasing the rate of tacking but also displacing some of the surface treatment into the low lying areas of the toner surface reducing the effectiveness of the surface treatment. Other devices rely upon toner-toner collisions in a fluidized bed to disperse the surface treatment. These collisions produce much lower shear and are more effective in achieving uniform dispersions. However, the normal forces are also lower and tacking is difficult to obtain.
The collision energy required to tack the surface treatment may be reduced by increasing the temperature of the fluidized bed. At elevated temperature, less kinetic energy from collisions is required to generate sufficient heat at the contact point with the surface treatment to exceed the Tg. Untacked but well dispersed surface treatment may be tacked under no shear by heating the toner. The attractive forces between the surface treatment and the toner core particle will cause the core material to plastically deform when near or above the Tg. Given sufficient time to increase the contact area between the surface treatment and the core material to the point of tacking but not to engulf the particle, a uniform tacked surface treatment can be obtained. The surface treatment prevents the toner from fusing together and the few points of surface treatment contacting two core particles are easily broken by sieving and subsequent action in the developer station.
Tacking of the surface treatment may be obtained in a wide variety of devices from sheared bed devices such as a Cylcomix (Hosokawa Micron Powder Systems) with no heat to static beds in ovens. Other devices that form beds and powder clouds with particle-particle collisions can provide tacking when appropriate heat is applied. Devices that form powder clouds with high collision energies such as jet mills or forced vortex classifiers (100ATP from Hosokawa Micron Powder Systems) need little or no heat for tacking of the surface treatment. Stirred bed devices such as Henschel mixers require temperatures ranging from 15° C. less than the Tg to the Tg depending upon the intensity of the mixing, the size and density of the toner, and the chemistry of the surface treatment.
Providing a well-dispersed surface treatment to separate the core toner from other surfaces may cause other problems. One problem is the reduction in the frequency of contacts with a charging surface such as a developer roll doctor blade in single component developers or a carrier in two component developers. The reduced contact frequency decreases the charging rate toner. To compensate, the surface treatment is modified with a chemical surface treatment to enhance charging of the particulate surface treatment. The charging of three component systems is poorly understood.
In the fully tacked state, charging of surface treated toner becomes a two-component system with a very heterogeneous toner surface. Initially, tribocharging is dominated by the surface treatment charging against the carrier or doctor blade. As surface treatment is embedded by mixing in the toning station, the dominant tribocharging mechanism transitions to that of the core toner charging against the carrier or doctor blade. Because of the transition in dominant charging mechanism with increased embedment, the charge level and humidity sensitivity varies with the degree of embedment. These changes in charging behavior may directly affect packed density through electrostatic forces and decrease the packed density due to increased Van der Waals forces at smaller separations for higher embedment. Higher Van der Waals forces between particles result in a less free flowing powder and thus decreased bulk density when the toner surface treatment has become embedded.
The average degree of embedment varies with the residence time of the toner in a process. The longer the toner is in a process, the more collisions it undergoes and the greater the embedment. The residence time varies in a toning station is inversely proportional to the image content of the documents being printed with that toner. As a result, the tribocharging properties may vary significantly with customer job stream.
The tribocharging becomes more complex for toners with surface treatment in the untacked state. Three two-component tribocharging mechanisms must be considered: core toner charging against the carrier or doctor blade, surface treatment charging against the carrier or doctor blade, and charging between the core toner and surface treatment. The surface treatment may be transferred to the carrier or doctor blade leaving a charge on the toner. It may also be back transferred to the toner leaving behind a charge on the carrier or doctor blade. Rapid transfer of surface treatment between toner and carrier may facilitate rapid charging. Transfer of surface treatment between toners particles may facilitate rapid charge transfer between toners increasing the charge of the lower toner while reducing that of the higher charged toner.
Two mechanisms of rapid charge transfer are possible. First, the mobility of the surface treatment provides mobility to the charge itself. Second, the kinetics of charge exchange may be increased resulting in a reduced propensity of the replenishment toner to dust out of the developer. The rate of charge exchange tends to be faster for components that are close to one another in a triboelectrification ranking. The chemistry of the surface treatment may be adjusted so that its tribo-level in a triboelectrification series is in between that of the toner and the carrier or doctor blade. When this is the case, the mean time of charge exchange between the toner and the surface treatment plus that between the surface treatment and carrier or doctor blade is less that that between the toner and carrier of doctor blade. Tacking and embedding the surface treatment will negate this rapid charge transfer.
One of the purposes of separating the toner surface from another surface is to prevent mass transfer of toner material to the charging surface of the carrier or doctor blade. As mass is transferred to the charging surface, it becomes closer to the toner in the triboelectrification properties and both the charge rate and level decrease resulting in a increased propensity of the replenishment toner to dust out of the developer. Two extremes of this mass transfer exist. When the surface treatment is highly embedded, the core material will be transferred and tribocharging is dominated by the differential rate of transfer of components from the core. At the other extreme is when excess surface treatment is used so that the core toner never contacts the carrier or doctor blade surface. At these levels of surface treatment, it is difficult for all of the silica to become tacked and surface treatment transfers to the charging surface. Because of the high level of surface treatment on the toner, back transfer of surface treatment to the toner is slow until the surface concentration of surface treatment on the charging surfaces approaches that of the toner. In this state, most of the collisions occur between surface treatment on the toner and surface treatment on the carrier or doctor blade and no charge is exchanged. Mass transfer of chemically reactive components from the core toner may also result in the loss of charging ability by the carrier or doctor blade.
Another purpose of separating the toner surface from another surface is to reduce the Van der Waals and electrostatic forces between the toner and the charging surface. Reduction of these forces allows greater exchange of toner to enhance charging rate and greater development rates of the image. The degree of separation must be controlled so that the attractive forces are greater then mechanical forces to prevent dusting from the development station.
Yet another purpose of separating the toner surface from another surface is to modulate the adhesive and cohesive forces in transfer of the toned image. Theses forces are minimized at a high degree of separation with uniform surface treatment. As the surface treatment is embedded these forces increase. At low forces, the transfer from the photoconductor to intermediate and final receivers is enhanced. However, the reduction in cohesion between toner particles allows the repulsive forces of the charge on the toner to push the toner particles apart upon transfer. The result is a large extent of dot explosion in halftone images and in satellites in text images. Embedment increases the adhesive and cohesive forces improving dot integrity and reducing satellites but reduces transfer efficiency. The variability in transfer of halftone and continuous tone images is increased and becomes visible as granularity. The high degree of variability induced in the state of the surface treatment by variability in toner residence time in the toning station as the image content varies leads to inconsistent image quality.
U.S. Pat. No. 5,066,558 teaches the use of a three-step process first to disperse a silica powder on a resinous core toner particle in a lower energy device, second to embed the silica in a second higher energy device such that there are little or no visible silica particles on the surface by SEM, and third to disperse additional silica powder in a device similar energy to that used in the first step. The method pertains to developers of 100 wt % toners and as such does not address issues of toner concentration control.
U.S. Pat. No. 6,087,057 teaches the use of two treated silica powders where the first silica powder is treated with an alkyl silane and an amino alkyl silane to give a negative charge and the second silica powder is treated with an organopolysiloxane that charges positive relative to the first and an third metal oxide to adjust charge. These formulas are selected solely for tribocharge stability upon admix, relative humidity changes, etc.
An object of the present invention is to provide a toner that mitigates print image degradation due to poor developer performance caused by changes in the degree of surface treatment embedment.
It is an object of the invention to provide toner with rapid mixing and charging resulting in developers that have low dust and provide uniform images resulting in reduced maintenance and service costs.
It is another object to provide toners that resist transfer of components from the toner to the surface there by providing long developer life, developer flow stability, and stable toner concentration control.
It is another object to provide toners that resist changes in the degree of surface treatment embedment with changes in residence time caused by changes in image content of print jobs.
It is a further object to provide means by which the charging level and the balance of charge and forces surface may be independently optimized using a single processing step during surface treatment.
These and other objects of the invention are described below.